Issue #3605990 by piotr pakulski: Prevent overlapping sticky action bars.

Apply file-browser-actions to either the multi-step selection display or widget actions, not both, so fixed footers no longer cover each other in the modal/iframe. The overlap is especially visible with the Gin admin theme, which styles the selection entities-list as a taller grid and uses larger status messages inside the Entity Browser iframe.

Closes #3605990